AI Connections dlg - add external .gguf file path option

This would be for non-portable versions only.  A possible UI solution might be a checkbox to select an external drive + it would then show a file open dlg that saves the path in the textbox. Would need to be a fast drive - e.g. PCIe 5.0 NVMe M.2 SSD

<img width="596" height="380" alt="Image" src="https://github.com/user-attachments/assets/19712391-bcf4-4aaa-81e8-56ce6b9a6eef" />